Octaoxygen, also known as ε-oxygen or red oxygen, is an allotrope of oxygen consisting of eight oxygen atoms. This allotrope forms at high pressure and remains stable across a wide pressure range at room temperature (between ). If it is further compressed past 96 GPa, then it transitions into a metallic, superconducting state.
The material is dark red and unlike other forms allotropes of oxygen is has no magnetic character, due to its unpaired electrons being forced to overlap.
Preparation and properties As the pressure of oxygen at room temperature is increased above , it undergoes a dramatic phase transition to the -phase allotrope. Its volume decreases significantly, and it changes color from sky-blue to deep red. This ε-phase was discovered in 1979, but the structure was unclear. A crystal lattice of tetraoxygen units was suggested in 1999, based on infrared spectroscopy. However, in 2006, two independent groups showed by X-ray crystallography that the structure was a rhomboid () cluster consisting of four molecules.
In this phase, it exhibits a dark-red color, very strong infrared absorption, and a magnetic collapse. It has been shown to have a monoclinic C2/m symmetry, and its infrared absorption behaviour was attributed to the association of oxygen molecules into larger units. At , the intra-cluster bond length of the cluster is , and the inter-cluster distance is , both longer than the bond-length in the oxygen molecule (). It is stable over a very large pressure domain, from to . Above , ζ-oxygen forms which is metallic and superconductive; ε-oxygen is non-conductive.
The formation mechanism of the cluster found in the work is not clear yet, and the researchers think that the charge transfer between oxygen molecules or the magnetic moment of oxygen molecules has a significant role in the formation.
Potential applications Liquid oxygen is already used as an oxidant in rockets, and it has been speculated that octaoxygen could make an even better oxidant, because of its higher energy density.
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